The need for an automation in small lot production where a large variety occurs between lots, has increased year-by-year and production lines have been automated by robots having a flexibility in dealing with various kinds of tasks and making frequent change between tasks. Automation of assembly processes for heavy articles with robots has been slow because any advantage gained by automation is small in comparison with the cost of a robot suitable for handling heavy articles.
Therefore, as shown in Japanese Patent Applications Nos. 160789/80 and 173016/81, assembly systems for heavy articles have had to be developed. These systems economically use an inexpensive balancer in conjunction with a robot of the type normally used for handling light loads. Typically in such systems, a rotary member of a potentiometer is connected to an operation stick and a conventional balancer controlled by a velocity instruction is instructed to rotate the rotary member through a particular angle. The operational stick is mounted on the tip of the arm of the balancer and rotated by a human operator. There is a problem with such arrangements, however, because the response characteristic of a robot and a balancer and between various kinds of robots are different from each other, particularly in the mechanisms and methods for driving the robots. Therefore, when a balancer is operated in cooperation with a robot, there is no assurance that the balancer and the robot will move in the same way in response to the same velocity instruction. Excessive forces operate at the junction of the balancer and the robot to restraint displacement of the balancer relative to the robot because of differences in relative velocities occuring due to the difference between their response characteristics.